A conventional method for measuring the concentration of an antigen in an analyte such as urine may utilize the combination of an antigen-antibody reaction and an optical technique. In this method, for instance, an analyte is mixed with an antibody for causing an antigen-antibody reaction, and the concentration is computed based on the absorbance outputted when the reaction system is irradiated with light. In this method, as long as the antigen concentration in the reaction system does not exceed a specific range, the measured absorbance with an increasing concentration of the antigen. However, when the antigen concentration in the reaction system exceeds the specific range, a phenomenon (prozone phenomenon) is observed wherein the measured absorbance decreases with an increasing concentration of the antigen. Such a phenomenon occurs not only in a antigen-antibody reaction but also in biochemistry in general. (Hereinafter, this phenomenon is referred to as “prozone-like phenomenon” inclusively of those in biochemistry.)
In a system in which the prozone-like phenomenon occurs, the computed concentration of the target substance may be lower than the actual concentration due to lowered absorbance. To eliminate such a disadvantage, the absorbance need be measured again after the analyte is diluted, for example.
However, to perform the measurement of the same analyte a plurality of times is not desirable in view of the high cost of the antigen. Further, a smaller number of times of measurement is preferable for facilitation of the measuring process. Although the prozone-like phenomenon does not occur in the measurement of e.g. the glucose level in urine, such measurement suffers from another problem that measurement resolution is poor in a high concentration range. Therefore, a system in which the prozone-like phenomenon does not occur also suffers from low measurement accuracy in a high concentration range.